Yersinia pestis, the causative agent of plague, delivers a set of 6 Yop toxins that act within host cells to negate and modulate signaling pathways that orchestrate host innate defenses that are crucial for developing an effective immune response. This proposal focuses on the mechanism of YopM. We have found that YopM specifically causes a global loss of natural killer cells, and those NK cells remaining do not contain detectable mRNA for IFNg. Concomitantly, YopM causes down-regulation in macrophages of net mRNA expression for key proinflammatory cytokines. In this project, we will test the hypothesis that YopM acts directly on cells such as macrophages and dendritic cells to prevent expression of cytokines such as IL- 15 and IL-12 that synergize to activate effector cells such as macrophages, polymorphonuclear neutrophils, natural killer cells, and cytolytic T cells for controlling numbers of Y. pestis. In Aim 1, we will determine whether YopM's action is mediated by hypothesized key cells by comparing the infection dynamics for virulent and YopM- Y. pestis (viable numbers, dissemination as a function of time) and host response (cell populations and cytokines) in primary pneumonic plague and bubonic plague in mice ablated for the cells genetically or by manipulation. In Aim 2, we will test our hypothesis that the Th1 cytokine and chemokine pathway mediates different host responses to YopM+ and YopM- Y. pestis between d 1 and d 3 to 4 p.i. in bubonic and pneumonic plague by infecting knockout mice or mice ablated for hypothesized key cytokines and analyzing the resulting infection dynamics and host response. In Aim 3, we will map the signaling circuitry of YopM's pathogenic effects by comparing transcriptional profiles (cDNA microarrays) and analyses of intracellular cytokines in purified macrophages from mice with pneumonic or bubonic plague due to virulent and YopM- Y. pestis. The data will reveal the pathogenic mechanism of YopM by identifying cells, cytokines, and signaling pathways modulated by YopM during plague. The findings will provide a completely new understanding of the biology of plague from both microbial and host perspectives as well as provide a framework for rational enhancement of resistance to plague in people threatened with exposure to plague. The data will reveal immune deviation points during plague and could identify potential therapeutic intervention targets. The findings also will provide a better understanding of mammalian innate defenses and insight into evasive mechanisms of pathogens and will facilitate the defeat of important human pathogens that are not select agents.